The present invention relates to an electron beam exposure apparatus and, more particularly, to an electron beam exposure apparatus which illuminates a mask with light emitted by a light source, photoelectrically converts the light patterned by the mask using a photoelectric converter, and exposes the object to be exposed using the patterned electron beam coming from the photoelectric converter.
Conventionally, optical steppers with high productivity are used in the mass-production process in the manufacture of semiconductor memory devices. However, in the manufacture of memory devices such as 1G or 4G DRAMs which have a line width of 0.2 .mu.m or less, electron beam exposure with high resolution and productivity has gained widespread appeal as one of exposure techniques that replace optical exposure.
Since conventional electron beam exposure normally uses a single beam Gaussian method and a variable forming method and has a low productivity, it is used in only applications that can make use of the feature of high resolution performance of an electron beam such as mask drawing, research and development of VLSIs, and exposure for ASIC devices to be manufactured in small quantities.
As described above, how to improve the productivity is the bottleneck to mass-production of exposure in an electron beam exposure apparatus. As a method of removing such a bottleneck, the following method has been proposed. In this method, a light beam is shaped into a predetermined pattern via a mask, and the patterned light beam is irradiated onto a photoelectric converter. An electron beam having a predetermined pattern and coming from the photoelectric converter is accelerated and converged, and is projected onto a wafer.
An example of a conventional electron beam exposure apparatus that realizes this method will be described below with reference to FIG. 12. This electron beam exposure apparatus 1 comprises, as an ultraviolet light source 2, a mercury lamp for emitting ultraviolet rays as a light beam. A collimator lens 3 and the like are inserted on the optical axis of the ultraviolet light source 2 to form an illumination optical system 4 there. A field angle limit aperture stop 5 for limiting the range of transmitted light, an ultraviolet mask 6 for shaping the transmitted light into a predetermined pattern, an ultraviolet projection optical system 7 for converging the patterned ultraviolet rays, a vacuum window 8, a photoelectric converter 9 located on the optical image plane of the convergent ultraviolet rays, and the like are disposed in turn along the optical axis of the illumination optical system 4. Since the photoelectric converter 9 emits electrons upon irradiation of ultraviolet rays, an acceleration electrode 10 for accelerating electrons, an electron projection optical system 11 for converging the electrons, a wafer 1000 located on the optical image plane of the convergent electrons, and the like are disposed in turn along the optical axis of the electrons emitted.
The wafer 1000 is held on a wafer stage 13 via a chuck 12, and the wafer stage 13 moves the wafer 1000 in two-dimensional directions perpendicular to the optical axis, which directions agree with back-and-forth and right-and-left directions in FIG. 1. The ultraviolet mask 6 is supported by a mask stage 14, which similarly moves the ultraviolet mask 6 in the two-dimensional directions perpendicular to the optical axis.
The vacuum window 8 consists of quartz glass, and is fitted in a vacuum chamber 15, and members below the photoelectric converter 9 are disposed inside the vacuum chamber 15. The ultraviolet projection optical system 7 is made up of a pair of optical lenses 16 and 17, aperture stop 18, and the like, and the electron projection optical system 11 is made up of a pair of electron lenses 19 and 20, correction coil 21, electron beam aperture stop 22, and the like. The electron lenses 19 and 20, and correction coil 21 comprise electromagnetic coils, and electromagnetically control an electron beam. An electron beam intensity detector 23 comprising, e.g., a Faraday cup or the like is arranged beside the wafer stage 13, and a secondary electron detector 24 is arranged at a position opposing the wafer 1000 from obliquely above.
The ultraviolet mask 6 shields transmitted light in accordance with a pattern formed of a chromium film on a quartz substrate, and is similar to that used in conventional photolithography. The pattern is formed in correspondence with the projection magnification of the electron beam exposure apparatus 1. For example, when the projection magnification of the ultraviolet projection optical system 7 and the like is 1.times., and that of the electron projection optical system 11 is 1/4.times., since the projection magnification of the electron beam exposure apparatus 1 is 1/4.times., the pattern to be transferred onto the wafer 1000 is formed on the ultraviolet mask 6 in a 4.times. enlarged scale.
The photoelectric converter 9 is prepared by forming an anti-charging transparent conductor film of, e.g., ITO (indium-tin oxide) on the surface of a quartz substrate having a thickness of several mm, and forming a photoelectric conversion substance of, e.g., CsI (cesium iodide) having a small work function. The converter 9 emits electrons at a density proportional to the incident light intensity. A power supply circuit (not shown) is connected to the transparent conductor film, and a voltage of, e.g., -30 kV with respect to a lens barrel is applied to the film. The acceleration electrode 10 is formed into a net pattern, and is applied with a voltage having the same potential as the lens barrel. For example, the electrode 10 accelerates electrons emitted by the photoelectric converter 9 to 30 keV.
The electron beam exposure apparatus 1 with the above-mentioned structure can form the pattern of the ultraviolet mask 6 on the wafer 1000 by exposure. More specifically, the range of ultraviolet rays coming from the illumination optical system 4 as a collimated light beam is limited by the field angle limit aperture stop 5, and the limited ultraviolet rays are incident on the ultraviolet mask 6. The ultraviolet rays are shaped into a predetermined pattern by the ultraviolet mask 6, and the patterned ultraviolet rays form an image on the photoelectric converter 9 via the ultraviolet projection optical system 7. The photoelectric converter 9 converts the incoming ultraviolet rays into an electron beam, and the electron beam is accelerated by the acceleration electrode 10. The electron beam then forms an image on the wafer 1000 via the electron projection optical system 11.
At this time, when the optical projection magnification is set at 1.times. and the electrooptical projection magnification is set at 1/4.times., as described above, since the total projection magnification is 1/4.times., the pattern on the ultraviolet mask 6 is reduced to 1/4, and forms an image on the wafer 1000, thus forming a micropattern. In particular, since an electron beam can realize finer exposure than ultraviolet rays, if the projection magnification of the electron beam is set to be smaller than that of the optical system, the pattern on the ultraviolet mask 6 can be satisfactorily reduced and formed on the wafer 1000 by exposure.
When the pattern on the ultraviolet mask 6 is formed on the wafer 1000 by exposure, as described above, since aberrations such as curvature of field and the like of the electron projection optical system 11 are produced, it is difficult to simultaneously form the entire pattern on the ultraviolet mask 6 onto the wafer 1000 by exposure. For example, when a micropattern as small as about 0.1 .mu.m is to be transferred, the region that can be transferred in one exposure is limited to that having a diameter of about 0.5 mm around the optical axis.
To solve this problem, in the above-mentioned electron beam exposure apparatus 1, the incident region of ultraviolet rays onto the ultraviolet mask 6 is limited by the field angle limit aperture stop 5, and the ultraviolet mask 6 and wafer 1000 are synchronously scanned and moved in the two-dimensional direction by the corresponding stages 14 and 13, thereby sequentially forming the entire pattern on the ultraviolet mask 6 onto the entire region of the wafer 1000.
In the above-mentioned electron beam exposure apparatus 1, by sequentially moving the region to be transferred from the ultraviolet mask 6 onto the wafer 1000 by exposure, the entire pattern on the ultraviolet mask 6 can be transferred onto the wafer 1000 by exposure at high resolution.
However, since only a portion of the ultraviolet mask 6 can be transferred onto the wafer 1000 by exposure at a time, several tens of reciprocal scans are required to transfer the entire pattern on the ultraviolet mask 6 onto the wafer 1000. As such scan movements are mechanically done by the stages 13 and 14, and high-speed operation is hard to attain, the conventional electron beam exposure apparatus 1 has a low productivity of exposure.